An addressable quantum dot qubit with fault-tolerant control-fidelity

Exciting progress towards spin-based quantum computing^1,2 has recently been made with qubits realized using nitrogen-vacancy centres in diamond and phosphorus atoms in silicon³. For example, long coherence times were made possible by the presence of spin-free isotopes of carbon⁴ and silicon⁵. However, despite promising single-atom nanotechnologies⁶, there remain substantial challenges in coupling such qubits and addressing them individually. Conversely, lithographically defined quantum dots have an exchange coupling that can be precisely engineered¹, but strong coupling to noise has severely limited their dephasing times and control fidelities. Here, we combine the best aspects of both spin qubit schemes and demonstrate a gate-addressable quantum dot qubit in isotopically engineered silicon with a control fidelity of 99.6%, obtained via Clifford-based randomized benchmarking and consistent with that required for fault-tolerant quantum computing^7,8. This qubit has dephasing time T₂^∗ =120 μs and coherence time T₂ 28 ms, both orders of magnitude larger than in other types of semiconductor qubit. By gate-voltage-tuning the electron g∗-factor we can Stark shift the electron spin resonance frequency by more than 3,000 times the 2.4 kHz electron spin resonance linewidth, providing a direct route to large-scale arrays of addressable high-fidelity qubits that are compatible with existing manufacturing technologies.